Method of welding metals and apparatus for use therefor

ABSTRACT

A method for welding parts using the thermo-electrical output of the metals to be welded to control and evaluate the welding process in which a single energy pulse, such as an electric current or a laser, containing a known amount of electricity is applied to the metals to heat the metals to be welded. Then the thermo-electric output of the metals to be welded is used to determine the mass of the metals. The energy necessary to effectively weld the metals is then determined as a function of the mass.

[0001] This invention relates to the welding of metals, and particularlythe welding of metals using the thermo-electricity output of the metalsbeing welded to control and evaluate the welding process.

BACKGROUND OF THE INVENTION

[0002] It is commonplace, in the manufacture of a wide variety ofelectronic and/or fabricated metal parts require spot welds to weldtogether two metals, and frequently metals which are dissimilar to eachother. One of the difficulties that has been incurred by the prior artin welding such metals is the inability to accurately control the powerused in the welding operation and hence the quality of the weld itself.It has been proposed, as described in U.S. Pat. Nos. 4,019,364,4,392,044 and 5,298,711, to either evaluate the quality of a weld or tocontrol the welding operation using the welded part as a thermocouple.

[0003] The techniques as described in the afore-mentioned patents sufferfrom the disadvantage of not being suitable to both control and evaluatein the same operation the welding process.

[0004] It is accordingly an object of the present invention to provide anew and improved method for the welding of metals using thethermoelectricity output of the metals being welded to both control andevaluate the welding process.

[0005] It is a related object of the present invention to provide amethod for the welding of metals in which the weld current is optimizedusing the resistance of the metals being welded and the rate of coolingof the metals being welded as a measure of the mass of the metals.

[0006] It is a more specific object of the Invention to provide a methodfor the welding of metals in which an initial pulse of current isapplied to the metals being welded to enable the resistance of themetals to be calculated and to enable the mass of the metals to bedetermined by the rate of cooling of the metals for use in thedetermination of the necessary welding energy required to effectivelyweld the metals.

[0007] It is yet another object of the invention to provide a method forthe control and evaluation of the welding of metals in which the rate ofcooling of the welded metals is used to evaluate the quality of theweld.

[0008] These and other objects and advantages of the invention willappear more fully hereinafter. For purposes of illustration and no oflimitation, the invention is described in the following drawings.

[0009]FIG. 1 is a schematic illustration of the welding system employedin the practice of the invention.

[0010]FIG. 2 is a schematic illustration of the welding power circuit,including, as a separate circuit, the computer analog output pulsecontrol circuit as illustrated in FIG. 2A.

[0011]FIG. 3 is a graphical representation of the electrical pulseemployed in the practice of the invention.

[0012]FIG. 4 is illustrative of the wave forms used in the evaluation ofthe welding operation.

[0013] The present invention is directed to a method and apparatus forwelding metals, and particularly dissimilar metals in which use is madeof the thermo-electricity output of the metals being welded to bothcontrol and evaluate the welding process.

[0014] In the practice of the invention, a known amount of thermalenergy is applied to the metals to be welded which serves to heat themetals to a temperature above ambient. The two metals together functionas a thermo-couple, and thus the thermo-electricity output of the metalsto be welded is measured to determine the rate of heat loss as thethermal energy imparted to the metals is dissipated. The rate of heatloss is a function of the mass of the metals to be welded, and thus thethermo-electricity output is a measure of the mass of the metals to bewelded. That mass is then used to determine the energy necessary toeffectively weld the metals. Using the energy necessary to weld themetals, it is possible to determine the optimum welding power necessaryto effect the weld.

[0015] In the practice of the invention, the energy pulse applied to themetals to be welded can be applied from a number of known energy sourcestypically used in welding; for example, the energy pulse may be a simpleelectrical current pulse or it can be the energy contained in a singlelaser beam. In either case, the energy supplied to the metals to bewelded serves to heat those metals, and the thermo-electricity output ofthe metals together functioning as a thermo-couple enables thedetermination of the rate of heat loss as a function of the mass of themetals.

[0016] The concepts of the present invention are applicable, asindicated, both to pulse laser beam and electrical resistance weldingtechniques, both of which are well known to those skilled in the art.When using electrical resistance welding, it is also possible to use apulse of electric current to determine the resistance of the metals tobe welded. In that situation, both the resistance level and the mass ofthe metals being welded are used together in determining the energynecessary to effectively weld the metals.

[0017] In accordance with another embodiment of the invention, it isalso possible, and sometimes desirable, to use the thermo-electricityoutput of the welded metals to evaluate the quality of the weld. In thatembodiment, the rate at which the thermo-electricity output of thewelded metals declines with time is determined as a measure of thequality of the weld. It has been found that a quality weld has as itssignature a rapid decline in that output voltage whereas a weld oflesser quality generally exhibits the signature of a more uniformreduction in the thermo-electricity output over time as the weldedmetals cool.

DETAILED DESCRIPTION OF THE INVENTION

[0018] An overall schematic of the welding system employed in thepractice of the present invention is illustrated in FIG. 1 of thedrawings. As shown there, two pieces of metal 10 and 12 are positionedin contact with each other where a weld is to be made. The two metalpieces 10 and 12 are thus positioned between a pair of contacts 14 and16. Included in the system is a weld pulse generator 18 and a computercontrolled analog output pulse control system generally shown as 20.

[0019] The metals to be welded can be any of a wide variety of metalswhich are either the same or different from the other. In general, themetals employed are those metals which can be welded by, for example,electric resistance welding and/or pulse laser welding as well as otherwelding techniques. For example, use can be made of welding techniquesknown to those skilled in the art as acetylene welding, heliac welding,TIG welding and arc welding along with electrical resistance and pulselaser welding.

[0020] The weld pulse generator 18 as illustrated in FIG. 1 is shown indetail in FIG. 2, and has been found to be particularly suitable whenusing electrical resistance welding. The circuitry contained in FIG. 2is known to those skilled in the art and forms no part of the presentinvention. That circuitry is known as an insulated gate bipolartransistor inverter circuit which drives the welding transformer.

[0021] The computer analog output pulse control is illustrated in FIG.2A of the drawings and is likewise a known circuit utilizing a computerto control the generation of the pulses.

[0022] In the preferred practice of the present invention, welding iseffected by the known technique of electrical resistance welding. In thepractice of the present invention, the metals 10 and 12 are subjected toan electric current pulse containing a known quantity of energy. Themagnitude of the pulse depends on a number of factors, including thenature of the metals to be welded as well as their total masses. Formost applications, pulses having a duration of 1 to 50 milliseconds andcontaining between 50 and 3,000 average amps have been found to bedesirable. It should be understood, however, by those skilled in the artthat larger welding jobs may necessitate longer pulse times and/orsignificantly larger current pulses. Typically the energy content ofthose pulses ranges from about 0.5 to 100 joules. Nonetheless, greateror lesser energy requirements may be appropriate depending on the sizeof the welding job to be performed.

[0023] When the single current pulse is applied to the metals beingwelded, it is generally desirable to calculate, knowing both the currentand voltage of the pulse, the resistance of the metals being welded. Inaddition, the application of the single current pulse containing theknown amount of energy serves to heat the metals being welded to atemperature above ambient. The contacts 14 and 16 also serve to monitorthe thermo-electricity output of the metals being welded. It has beenfound that the mass of the metals being welded is a function of the rateat which those metals cool which can be determined by measuring the rateof reduction in the thermo-electricity output of the metals duringcooling after the application of the single current pulse. That rate ofthermo-electric voltage reduction is then used to determine the amountof energy necessary to effectively weld the metals.

[0024] As an example, the energy pulse used in welding wires of twodissimilar metals, namely Kovar and nickel, is illustrated in FIG. 3 ofthe drawing. FIG. 3 is a plot of voltage on the vertical axis and timein milliseconds on the horizontal axis. The initial current pulse isillustrated in FIG. 3 as pulse A and may be a pulse of about 400 ampsapplied to the Kovar-nickel wires to be welded over a time of about 5milliseconds through the contacts 14 and 16 in FIG. 1.

[0025] The contacts 14 and 16 thus apply a known current pulse I havinga known voltage V to the wires to be welded. Using the conventionalequation that R=V/I, the resistance R of the wires can be calculated,typically using a computer. In addition, the voltage output of the weld,namely points B to C in FIG. 3, is also monitored. Because the two wiresto be welded function as a thermocouple, the rate of cooling betweenpoints B and C can be monitored by monitoring the voltage drop betweenpoints B and C as a measure of the mass. Using the familiar equationthat the energy in joules is equal to I²RT where R is the resistance andT is the time in seconds, the amount of energy in joules can bedetermined by the foregoing equation.

[0026] In the preferred practice of the invention, the welding currentis applied to the metals to be welded by a number of pulses betweenpoints C and D in FIG. 3. Thus the welding current pulses are preferablyapplied N number of times as determined by the energy required to effectthe weld. N is determined by the calculated mass and resistance of thematerials to be welded.

[0027] In accordance with the preferred practice of the invention, afterthe weld has been made by the application of N number of current pluses,the rate of cooling is then measured by the rate in decline of thevoltage generated by the welded metals serving as a thermo-couple. Thateffect may be illustrated in FIG. 4 of the drawing in which there isshown a plot of voltage versus time for the metals which have alreadybeen welded. The thermo-electricity output of the welded metalsillustrated as a voltage is shown by curve E which declines betweenpoints E and D. It has been found that there is a correlation betweenthe rate of decline in the thermo-electricity output of the weldedmetals which is directly related D the quality of the weld. In, forexample, the curve between points E and D, the rapid rate of voltagedecline, and particularly between points E and F, is indicative of agood weld.

[0028]FIG. 4 also illustrates the rate of thermo-electricity decline ofanother weld between points G and H. The gradual rate of voltage declineas measured by the contacts 14 and 16 in the weld is indicative of aweld of lesser quality.

[0029] Thus, the thermo-electricity output of the metals to be weldedand of the weld as effected can be used to control the weld as well asto evaluate its quality.

[0030] It will be understood that various changes and modifications canbe made in the details of procedure and use without departing from thespirit of the invention especially as defined in the following claims.

What is claimed:
 1. A method for welding metals using thethermo-electricity output of the metals to be welded to control andevaluate the welding process comprising the steps of: (a) applying tothe metals to be welded a single energy pulse containing a known amountof energy whereby the energy pulse heats the metals to be welded; (b)determining the thermo-electricity output of the metals to be welded todetermine the mass of the metals to be welded as a function of the rateof cooling; (c) determining the energy necessary to effectively weld themetals as a function of the mass of the metals as determined by thethermo-electricity output therefrom; and (d) applying the weldingcurrent containing the determined energy to effect the weld.
 2. A methodas defined in claim 1 wherein the energy pulse is an electric currentpulse.
 3. A method as defined in claim 1 wherein the energy pulse issupplied in a laser beam.
 4. A method as defined in claim 1 wherein theenergy pulse is a pulse of electrical current and the energy necessaryto effect welding the metals is determined by the mass of the metals andthe resistence of the metals.
 5. A method as defined in claim 1 whichincludes the step of measuring the thermo-electricity output of themetals after welding as a measure of the quality of the weld.
 6. Amethod as defined in claim 1 wherein the energy pulse is an electricalcurrent having a duration of about 1 to about 50 milliseconds.
 7. Amethod as defined in claim 6 wherein the electrical current pulsecontains about 50 to about 300 average amps.
 8. A method as defied inclaim 6 wherein the electrical content of the energy pulse ranges fromabout 0.5 to 100 joules.
 9. A method for welding metals using thethermo-electricity output of the metals to be welded to control andevaluate the welding process comprising the steps of: (a) applying tothe metals to be welded a single current pulse containing a known amountof energy whereby the current pulse heats the metals to be welded; (b)measuring the resistance of the metals to be welded as a function of thecurrent pulse and its voltage and measuring the mass of the metals to bewelded as a function of the rate of cooling as determined by the levelof the thermo-electricity; (c) determining the energy necessary toeffectively weld the metals as a function of the resistance of the metalto be welded and the mass of the metals as determined by thethermoelectricity output therefrom; and (d) applying a welding currentcontaining the determined energy to effect the weld.
 10. A method asdefined in claim 9 wherein the energy pulse is an electrical currenthaving a duration of about 1 to about 50 milliseconds.
 11. A method asdefined in claim 9 wherein the electrical current pulse contains about50 to about 300 average amps.
 12. A method as defied in claim 9 whereinthe electrical content of the energy pulse ranges from about 0.5 to 100joules.